Configurations of antennas producing multiple resonances are disclosed in the following patent documents.
For example, Japanese Patent No. 4129803 discloses an antenna having a radiation electrode (non-feeding element) which projects outside a ground area of a mounting board. In the antenna, non-feeding elements are formed using radiation electrodes on the top and bottom surfaces of the mounting board or using an independent radiation electrode.
Japanese Unexamined Patent Application Publication No. 2004-129234 discloses an antenna having a radiation electrode that extends along an edge of a mounting board.
Japanese Unexamined Patent Application Publication No. 2009-171096 discloses an antenna in which a feeding element and a non-feeding element are arranged in parallel.
In addition, PCT International Publication No. WO2007-043150 discloses an antenna having a feeding element provided on a rear surface of a casing and a non-feeding element provided on a side surface of the casing.
FIG. 1 illustrates an example of an antenna disclosed in Japanese Unexamined Patent Application Publication No. 2004-129234. A ground plate 21 is a conductive circuit board. A feeding point 22 is provided on a side edge of a ground plate 21 to feed power to an antenna element 23 connected to the feeding point 22. The electrical length of the antenna element 23 is approximately ⅜ wavelength of the operation frequency band. The antenna element extends from the feeding point 22 along an edge portion of the ground plate 21 within the thickness of the ground plate 21 and is short-circuited to the other side edge of the ground plate 21.
However, the antenna configurations described above have problems in measuring antenna performance in terms of efficiency, band width, and in improving the antenna performance.
In the antenna disclosed in Japanese Patent No. 4129803, the area of the second non-feeding radiation element is increased to improve antenna efficiency. The antenna has a structural limitation in that it is necessary to enlarge the mounting board to the outside or prepare an independent element in order to expand the area of the second non-feeding radiation element.
To improve the efficiency of the antenna disclosed in Japanese Unexamined Patent Application Publication No. 2004-129234, it is also necessary to enlarge the mounting board or prepare an independent radiation element, and such an arrangement is subject to a structural limitation.
To improve the efficiency of the antenna disclosed in Japanese Unexamined Patent Application Publication No. 2009-171096, it is necessary to increase the height of the antenna. In addition, a decrease in the width of the antenna (direction along which the feeding element and the non-feeding element are arranged) decreases the width of the radiation electrodes, which increases the probability of loss.
In the antenna disclosed in PCT International Publication No. WO2007-043150, a radiation electrode is arranged on a casing surface. Thus, when the radiation electrode is composed of a flexible substrate, copper foil or the like, the radiation electrode may become detached from the casing, and deviations and variations of attachment position may occur. As a countermeasure, a spring contact may be used to feed power. However, such an arrangement complicates the antenna structure.